Measure it after the bodywork, not after the tires
A trailer that has been repaired following a collision gets inspected for the things people can see. The panels line up, the lights work, the doors close, and it goes back into service.
What nobody measures is whether the axles are still where they were. An impact hard enough to need repair is an impact capable of moving a frame, and a trailer can come out of a shop looking correct and tracking several degrees off.
The tires find out first, months later, and by then the repair is long settled and nobody connects the two. Measuring a trailer after structural repair is a small job at the right moment and an unanswerable question at the wrong one.
The same applies after a hard dock strike, after a landing gear failure, and after anything that bent something underneath.
What gets measured
Axle alignment to the kingpin, the relationship between axles on a tandem or tri-axle, and the suspension and mounting hardware holding it together.
A trailer measured on its own answers where its axles are. Measuring it with the tractor answers why the combination tracks the way it does, and heavy-duty truck alignment is at this address.
Reading it from the tires
Wear that is opposite side to side on an axle, one edge going down on the left and the other edge on the right. A trailer running offset behind the tractor on a straight road. Fuel figures drifting on a route that has not otherwise changed. The same position being replaced every time.
Drivers will not report it. A trailer out of line transmits very little forward to the tractor.
In a fleet that swaps trailers
One crooked trailer wears whatever tractor pulls it, so the damage appears across several tractors and looks like several separate alignment problems.
Fleets then align tractors repeatedly and the wear returns. Including trailers in the measurement is what breaks that cycle.
The casing
A trailer position scrubbed on one shoulder will not pass at the retread plant on this site. Trailer casings retread well when they wear evenly. See retreading.
About this store
The Denver store carries trailer tires and every other Purcell category, and is managed by Jake Perry. Colorado Springs and Loveland also align trailers.
See the national trailer alignment page for all 36 locations.